The two halves of altimeter should give you a clue to its meaning: altitude is height above ground, and meters are used to measure distance. An altimeter measures altitude. This is an extremely important device in the field of aviation. When flying an airplane, pilots needs to know exactly what their altitude is: the altimeter tells them. Pilots flying helicopters, blimps, and other flying vehicles use altimeters too. A plane without a working altimeter isn't safe to fly.